I am very surprised, the negative wire is not always negative!

Pics or it didn't happen:

I clamped the diode in the same way that the wire is clamped in:

And the positive voltage readout:

 

You don't need four of them.

I think I do, your illustrations show 2 speakers but my car has 4. I have 8 wires. I thought you were just abridging your diagram, but maybe I didn't mention that in my first post.

The problem with having four switches is that you can switch them in the wrong order and end up shorting two outputs together.

Even having two DPDT switches you still run the risk of not switching things at the right time. Unless you plan on turning both amps off before you select which amp plays to the speakers.

And there's one more concern I have regarding the amp that is NOT connected to the speakers - isn't it hard on an amp when it has no load on it? This I don't know - I raise this question so that someone with more experience can answer it.

The only thing I could see happening from from having more than one DPDT switch is that I could accidentally have speakers 1 2 and 3 on amp 1 and speaker 4 on amp 2. I would have the + and - terminals of one speaker on the same switch, so switch 1 is both terminals of speaker 1, and so on.

Also, I won't be switching these very often, it would most likely only be when I need to listen to the radio for some reason.

And I don't think it is hard on the amp to run it with no load. The amp is designed for 4 ohm or greater speakers. It is compatible with 8+ ohm speakers but it can't put as much power into them. It puts out whatever voltage corresponds to its input voltage and its gain ratio. It uses a fixed gain (it is adjustable, but it stays fixed unless you're turning the screw in the side of the amp). The power output will drop to 0 with no speaker attached.

My speakers do have spade lugs and I soldered the wires directly onto those lugs.

 

What I would consider is using relays. This https://www.mouser.com/ProductDetail/Schneider-Electric/782XDXH21-12D is a 4PDT relay.

Using relays puts the power contacts near the amp which is likely in the trunk. There are various mounts for the relays and you can use FASTON terminals to make the connections. That one is a bit pricey.

DIN rail modules such as these https://www.asi-ez.com/member/x034-Single-Channel-4-Pole-Relay-Modules.asp offer screw terminals.

Wire ferrules such as these https://www.mouser.com/ProductDetail/Panduit/FSD77-10-D make better connections to stranded wire, No special tool is needed. the tube collapses around the wire and thus the screw doesn't push the stranded wires apart. Quality terminals will have a wire protector where a flat plate squishes the wire.

This http://www.winford.com/products/rlp104.php link shows DIN rail in action.

You can buy terminals such as https://www.asi-ez.com/member/x526-DIN-Rail-Terminal-Blocks.asp These usually require a body and an end cap terminal. An end plate can add separation. e.g. every 2 terminals.

End clamps https://www.asi-ez.com/member/x007-Terminal-Block-Accessories.asp prevent side to side movement. One at each end or a series of terminals/blocks.

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You can use a little known component known as an impulse relay which will keep the same state when powered off. Thus a single push button can "toggle" the selection.

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I didn't pick out anything for you, but I know it gets messy.

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Tony: Solid state amps are fine with the speakers disconnected. Tube or valve amps are not.

 

Tony: Solid state amps are fine with the speakers disconnected. Tube or valve amps are not.

Shows my age. No ? ? ?

 

@Tonyr1084 Tony, Do you know how to use one of these: https://en.wikipedia.org/wiki/Model_500_telephone

They seem to be bullet proof. Have two working in the house with 4-prong jacks. Original equipment from the 1960's. upgraded to 4-prong.

Most calls are made using a cordless phone with 4 stations and an "Answering machine"/base on a UPS. Both touch-tone and speaker phone seems to be a necessity.

 

@Tonyr1084 Tony, Do you know how to use one of these:

Yes. Sadly. And I agree, they ARE bullet proof.

Funny you should ask. The marble machine I'm building right now has three REAL BRASS BELLS that came from some of these old telephones. Mom never threw out anything she thought could have some value. Over the years I've kept those phones and turned the bells into wind chimes. Well, the marble machine I'm building has three of these bells installed. Spent about 90 minutes yesterday polishing up the bells and they look spectacular. Sound great too. Nothing compares to the real analog sound of a well made bell. Electronic bells ? ? ? PEW ! ! ! I love trains. I HATE the electronic bell sounds they now have.

Yeah, I grew up sticking my finger in the "Dial". That's where the term comes from "Let me dial her number." Even though we've had push buttons, speed dial and now "Hey Siri, call Jenny" and the phone automatically dials 8675 309. There ARE some things I'd LOVE to go back to. Analog stereo amps with tubes. Gaud they sounded great. My old Magnavox in the Cherry Wood cabinet with the 15 inch woofer, the 12 incher and the two horn tweeters. Of course, I must now attribute some of my tinnitus to that machine. That and my drums.

 

Yeah, I grew up sticking my finger in the "Dial"

The small town in Wisconsin I grew up in didn't convert to a dial system until I was in high school.
Our phone number was 187W and the other person on our party line was 187J.
You just picked up the phone and the operator said "Number please".
Looking back, I guess we rather had Siri way back then.

 

I guess we rather had Siri way back then.

Siri? Or was it "Sarah"? Was on Andy Griffith.